In a part-time four-wheel-drive vehicle, for example, only front axles are constantly connected with the transmission and rear axles are sometimes connected therewith. For the purpose of establishing connection of the rear axles automatically or in accordance with operation by a driver, a transmission device containing a clutch may be used, which may intervene between a propeller shaft and a rear differential for example.
In this device, what should be selected as an actuator for coupling the clutch is a technical problem. While torque sufficient for running the vehicle acts on the propeller shaft, the actuator device must overcome this torque to establish connection of the clutch. Even where a hydraulic device or an electric motor is used, the device used must output considerably great power when the power alone must establish connection of the clutch or retain the connection. The actuator inevitably requires a great scale and great energy consumption.
Japanese Patent Application Laid-open No. 2004-316893 discloses a related art.